SUPEROXIDE GENERATION AS A BASIS FOR PLATELET HYPER-AGGREGABILITY AND NITRIC OXIDE RESISTANCE IN PATIENTS WITH MYOCARDIAL ISCHAEMIA
A.S. Holmes*, Y.Y. Chirkov, J.D. Horowitz.
Cardiology Unit, Queen Elizabeth Hospital, Adelaide, South Australia.
We have previously observed that platelets from patients with stable angina pectoris (SAP) exhibit increased aggregation in response to ADP and are hyporesponsive to the anti-aggregatory effects of sodium nitroprusside (SNP) and other nitric oxide (NO) donors, irrespective of prior nitrate therapy. We now investigate the possibility that the observed phenomenon of NO resistance in platelets is associated with increased concentrations of superoxide radical (O2), which is known to inactivate NO.
Methods: In blood samples from normal subjects (n = 18) and patients with SAP (n = 22) or acute coronary syndromes (ACS, n = 16) we studied inhibitory effects of SNP (10 mM) on ADP (1 mM) induced platelet aggregation (impedance aggregometry) in conjunction with an O2 assay (lucigenin-enhanced chemiluminescence). Specificity of the O2 detection was verified with superoxide dismutase in combination with catalase (SOD/Cat).
Results: Platelets from patients of both groups were more aggregable (p<0.01) and less responsive to anti-aggregating effect of SNP (p<0.01) than platelets from normal subjects. O2 contents (chemiluminescence signal, mV; mean±s.e.m.) are summarised in the table:
|
O2 CONTENT |
NORMALS |
SAP |
ACS |
|
Before aggregation |
62±8 |
125±20 |
280±60"# |
|
After aggregation |
200±50 |
660±80" |
830±180" |
(p<0.05 and "p<0.01 vs Normals; #p<0.05 for ACS vs SAP)
Platelet responsiveness to SNP was inversely correlated with both basal (.~=-0.83) and post-aggregation (.~=-0.99) O2 content. Among patients with SAP, higher O2 levels were recorded in those with 3-vessel stenosis (p<0.05) than in those with 2- or 1-vessel stenosis. SOD/Cat normalised responses to ADP (p<0.01) and SNP (p<0.01) in patients, with no effect in normals.
Conclusion: Increased O2 concentration in blood of patients with both SAP and ACS contributes towards platelet hyperaggregability and hypo-responsiveness to anti-aggregatory effects of NO donors.
